Conventional shot peening or blasting is often used to treat concrete or metal surfaces by blowing or mechanically impelling spherical particles of steel or iron against the surface. For example, new concrete surfaces often must cure for approximately thirty days, after which the top layer of concrete is removed to eliminate latents, residuals, and other undesirable characteristics of the top layer of the concrete. When the top layer is removed, a uniform surface is provided, which allows for more effective application of coatings and the like. Similarly, it is often desirable to remove existing surface coatings, such as paint, prior to the application of a new coating. Shot peening may be used for either of these purposes, because the impact of the individual particles tends to loosen and dislodge particles of concrete and coating.
For metal surfaces, the individual particles produce shallow, rounded overlapping dimples in the surface, stretching the surface radially from each point of impact and causing cold working of the material. The resultant compressive stresses tend to counteract tensile stresses imparted to the workpiece by other processing steps, such as rolling, bending, or abrading. Such blasting or peening operations thus have wide applicability in the field of surface treatment.
Conventional shot peening, although useful for some purposes, has certain disadvantages that tend to limit its usefulness. For example, large and expensive equipment is typically required for collecting, screening, and recirculating the shot particles and impelling them against the surface of the workpiece. Such blasting equipment typically is not portable, and thus may only be used with those workpieces that can be brought to the blasting equipment. Shot peening a workpiece while it is attached to a piece of equipment is very difficult because the particles cannot be easily collected. Shot peening a surface such as a concrete highway is also quite difficult, because the blasting equipment is not highly mobile, and the particles are not easily collected for recirculation.
An alternative to shot peening processes described above is the use of rotary peening devices such as those described in U.S. Pat. Nos. 3,648,498 (Voss et al.), 3,834,200 (Winter), and 3,857,750 (Winter et al.). Rotary peening devices have been used in processes such as the removal of residual stresses in metals, removal of scale and coatings on metal surfaces, removal of coatings on concrete surfaces, and surface preparation of concrete for new coatings. In such rotary peening tools, the peening action is provided by peening rivets attached to peening flaps that are retained by and extend from the perimeter of a hub. As each successive support flap is rotated in proximity to the surface, the peening rivets contact the surface and simulate the peening action of the individual spherical particles of conventional blasting. The rotary peening apparatus is portable, and the peening rivets eliminate the need for collecting and recirculating individual particles.
A rotary peening apparatus 10 according to the prior art is shown in FIG. 1. Apparatus 10 includes a cylindrical hub 12 including hub ends 14 and 16 and opposed hub mounting flanges 18 and 20, only one of which is shown. The hub is adapted for mounting on a shaft or arbor (not shown) for rotation therewith about central axis A--A. Hub 12 includes a plurality of guides 22 that are spaced about the perimeter of hub 12 to provide flap slots 24, which extend parallel to axis A--A and are adapted to receive retaining end 26 of peening flaps 30. Disposed within retaining end 26 is a keeper pin 17, which assists in retaining peening flap 30 within flap slot 24. Several flaps are typically retained in side-by-side relationship within each flap slot, although only one such flap is shown in FIG. 1 for simplicity. Retaining ends 26 of peening flaps 30 are adapted for sliding engagement with flap slots 24, such that the peening flaps may be removed and replaced as desired. For example, a peening flap may become worn or broken, or a different type of peening flap may be required for a particular application, at which time one or more peening flaps may be removed by sliding the peening flaps out of the flap slots 24.
Peening flaps 30 also include a peening end 32, as shown in FIG. 2, which holds one or more peening rivets 34, 36, 38, and 40. Each peening rivet includes a plurality of peening particles 42 protruding therefrom, which particles impact the surface 50 when apparatus 10 is rotated in sufficient proximity to surface 50, as shown in FIG. 1. The peening rivets simulate the individual particles of the blasting processes of the prior art, and may be constructed of, for example tungsten carbide shot. The shot is brazed to a hardened rivet, which is fastened to a flexible flap material to form peening flap 30.
One potential disadvantage of such a rotary peening apparatus occurs if each peening rivet is circumferentially aligned with each successive peening rivet. Because the peening rivets are spaced across the face of the peening flaps, the area of the surface 50 that is between adjacent peening rivets would not be contacted by a peening rivet, and therefore may not be appropriately treated. Such an arrangement can result in a surface having a plurality of ridges and troughs, known as a grooved surface. A grooved surface finish may have certain advantages, such as slip-resistance and water drainage, but for other applications it is more desirable to produce a relatively uniform surface finish. For example, it may be advantageous to provide a uniform surface for receiving paint or other coatings.
In a previous attempt to form a relatively uniform surface, the peening flaps of the rotary peening apparatus have been offset with respect to the peening flaps in adjacent flap slots by one-half of the distance between peening rivets. For example, a hub may include twenty flap slots, and five peening flaps within each flap slot. A spacer ring 60 such as that shown in FIG. 3 includes pin members 62 having a length equal to one-half of the distance between adjacent peening rivets. The pin members 62 are located so as to slide into alternate flap slots 24 around the circumference of hub 12, thereby offsetting alternate rows of flaps with respect to the hub end. Because the length of the pin members is equal to one-half of the distance between peening rivets, each row is offset with respect to each adjacent row of peening flaps. By offsetting alternate rows of peening flaps, the surface finish produced by the rotary peening apparatus is more uniform than would be produced by an apparatus where each peening rivet was circumferentially aligned with each succeeding peening rivet.
Peening apparatuses having alternate rows offset, although an improvement over peening apparatuses wherein each peening rivet is circumferentially aligned with the peening rivet on each adjacent flap, tend to produce a surface having small grooves or streaks in the surface finish. The effect is similar to the case where no spacer is present, because each peening rivet is circumferentially aligned with the corresponding peening rivet on alternate peening flaps around the periphery of the apparatus. Thus, although a spacer for offsetting alternate peening flaps may improve surface finish by ensuring that not all peening rivets are circumferentially aligned, a streaked surface may still result. Such a surface can be undesirable because multiple passes of the peening apparatus may be required to peen the surface adequately. It is therefore desirable to provide a rotary peening device that will produce a relatively uniform surface finish compared to the rotary peening apparatuses of the prior art.